Permanent magnets consist of a ferromagnetic material that has been suitably magnetized. Permanent magnets provide a magnetic field without requiring an electric current. Permanent magnets are attracted to and can be firmly affixed ferromagnetic materials. However, permanent magnets suffer from the drawback that their energy output is fixed and they cannot be employed if a variation in magnetic field strength is required.
Due to the above-mentioned drawback with permanent magnets, it has been customary to control magnetic fields by using electric currents or electromagnets. The strength and orientation of the magnetic fields of electromagnets can be controlled successfully by controlling the strength and direction of the electric current However, this introduces the necessity of having to provide a constantly available source of electric current and associated wiring. This can cause complexity and potential hazards.
A number of attempts have also been made to arrange permanent magnets to enable variations to be achieved in magnetic field strength. A number of different approaches have been tried to produce practical solutions. However, these approaches resulted in bulky constructions, complicated constructions, or devices that were expensive to manufacture. The prior art devices also paid little attention to achieving a good holding force between the devices and substrates under conditions where an incomplete external magnetic circuit existed, for example, where there were gaps between the magnet and the keeper, or where the keeper or substrate had a low permeability, such as thin ferrous sheet metal. To achieve good holding forces in the prior art devices, it was necessary that perfect contact between the magnet and the substrate be achieved and that the substrate be of substantial volume to enable a sufficient magnetic flux to be established. This, of course, could be difficult to achieve in practice.
U.S. Pat. No. 3,121,193 (Engelstead) describes a permanent magnet-type work holding device especially a chuck that is adapted for use in turning operations. The device of Engelstead consists of arrays of permanent magnets, which magnets are of generally rectangular configuration. In order to adjust the field strength an array of pole pieces was moved away from and out of alignment with another array of pole pieces.
U.S. Pat. No. 4,251,791 (Yanagisawa) describes a magnetic base which is switchable between an engaged position and releaseable from a substance by an operation of a single rotating permanent magnet disposed notably in a magnetic circuit block. The rotary magnet in Yanagisawa is rotated by 90° to switch between the exited and the unexcited states. This device relies upon using passive external magnetic members of substantial mass (12 and 14 of FIG. 1 of Yanagisawa) having a rotatable magnet (20) positioned therein.
United Kingdom patent application no 2130797 relates to a permanent magnet chuck capable of switching from an excited state to an unexcited state by changing the relative positional relationship between fixed permanent magnets and movable permanent magnets. The movable permanent magnets move laterally with respect to the fixed magnets in order to vary the magnetic filed.